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May 30, 2007

Footing Drains

My desert location receives approximately 15-18" of rain per year (if we're lucky!). In the summer monsoon season, heavy rain falls during thunder-showers. The key to keeping water away from the studio will eventually be proper finished grading of the soil, in order to slope away from the stone walls.

This location does not have issues with waterlogged soil like other parts of the country; where collected water between the foundation wall and the soil is subject to kind of a hydraulic pressure that in effect, forces the water through cracks in porous foundation walls. However, even in the dry desert, building codes and good practice recommends installing a provision for water removal around the footings/foundation of most structures This prevents water from seeping through walls into living areas and water saturated soil from undermining the structural integrity of the footing/foundation.

The first step was to waterproof the stone wall that will be buried in the berm. I decided to try an elastomer (rubberized) roof coating. I applied two coats (fill & finish). In the old days, I used 'tar' to waterproof foundations, but I'm trying to avoid petroleum products in this structure; and also, tar is very messy. Will the elastomer coating work for the long haul? I don't know, but the instructions said it can be applied to stone & masonry. It has a 10-year rating for roofs; and the coating won't be subject to UV sun damage as it will be buried in the ground.

As the photo above shows, the ground was leveled and on it was placed a 6-mil plastic ground sheet to prevent the clay soil base from clogging the pipe. The plastic pipe is perforated, and the holes were placed down to draw-in any water collecting on the plastic. The pipe was leveled. To get around the circular shape of the building I used 45-deg elbows and connected short sections of pipe (as illustrated in the photo).

The photo to the right is kind of a cross-section of the footing drain project, showing gravel around and on top of the pipe, followed by a top sheet of plastic to keep the clay soil that will be used for back-fill, from working its way down into the gravel over time and clogging pipe (click photo to enlarge).

Also in the photo you can see rigid foam insulation (R2.9), applied against the stone stem-wall. This insulation will provide a thermal break between the earth berm and the building.